The field of isolating cells, virus, bacteria and soluble molecules has used various types of particles as a solid phase to absorb or bind to the target of interest. For example, magnetic particles, when coated with a ligand, such as an antibody, can bind to a target cell or soluble protein. The bound target on the magnetic particle effects a separation of the target from other cells types or proteins. There have been various improvements on this original work and commercial examples have been available for years.
Others have used latex particles, liposomes, milk fat globules, plastic particles such as polystyrene and polyethylene and polypropylene, nylon etc. As capture supports, each of these has its own particular attributes and problems. Non-specific binding of non-target cells and proteins is the most common problem encountered in these methods, resulting in imperfect separation. Additionally, most methods require at least one additional step following binding to effect separation of unbound species from the particle-bound, e.g. for magnetic particles a magnetic field must be applied, centrifugation is sometimes used to separate the particles from solution or filtration of the particles from solution.
In general, the time required to bind the target cells or proteins is related to the surface area of the particles and the quantity of particles per unit volume of solution. The smaller the particle the more rapid the binding due to increased surface area. Unfortunately, greater surface area generally increases non-specific binding.
Separation methods may, depending on the nature or principle of the separation, co-isolate different particles. This is apparent in separations based on gravity centrifugation in which particles and cells or other species may co-pellet. In addition some cell types such as macrophages and monocytes may themselves non-specifically ingest the particles and can be isolated along with the target cells. While individual limitations of the previous technology may be minimized or avoided by taking certain steps or precautions, certain limitations are inherent and cannot be entirely overcome. The fact that there are many different approaches with varying degrees of success suggests that a better solution to the problem is needed.
The optimal separation agent would have an infinite surface area with zero non-specific interaction so that the binding would occur instantaneously and minimize binding to non-target cells or soluble molecules. Ideally, the agent should separate itself from the cell suspension or soluble molecule solution without entrapping non-target cells or molecules respectively.